The present invention relates to electrochemical sensors and, more particularly, to methods for improving bandspreading and improving the signal-to-noise ratio in electrochemical flow through sensors. The invention has particular utility with electrochemical sensors of the coulometric type and will be described in such utility.
Electrochemical flow through sensors are well established in the art as useful analytical tools, particularly for, although not limited to, detection and determination of trace impurities in water, or the detection and determination of trace compounds in biological samples or the like. Recently, flow sensors of essentially 100% efficiency or coulometric type (as opposed to amperometric sensors with an efficiency of 3-5%) as typified by U.S. Pat. Nos. 4,404,065; 4,233,031; 4,413,505; 4,497,199 and 4,511,659 (all of which are assigned to the common assignee of this application) incorporating a series of porous conductive detector or working electrodes have been shown to have several unique applications in industrial and biomedic problems.
A typical sensor of this type is shown in simplified form in FIG. 1. The sensor, generally indicated at 10, comprises a series of cells, generally indicated as 12, which define a path for the flow of test fluid 14 therethrough between the inlet at 16 and the outlet at 18. While a single cell 12 can be used, it is common to place a number of cells 12 in series to develop a larger electrical signal to more accurately reflect the characteristics of the test fluid 14. Each cell 12 typically contains a porous detector electrode 20 across the path of flow of the test fluid 14 through the cell 12. It is the detector electrode 20 in each cell 12 which develops the electrical signal of interest as the test fluid 14 passes through it. As shown in the enlarged drawing of FIG. 2, the detector electrode 20 is comprised of a porous electrode base material 22 such as fritted graphite or fritted carbon with pores 23. The porous material 22, in turn contains intersticial spaces 24 in the underlying matrix of the porous material 22. Since it is virtually impossible to create a homogeneous fritt material wherein the intersticial spaces 24 are completely uniform throughout, the porous material 22 also may contain enlarged secondary pores 26. Characteristically, the fluid 14 will find an optimum path through the cells 12 of the sensor 10. These enlarged secondary pores 26 and interstitial spaces 24 can be a source of noise, particularly when they are out of the optimum path and, therefore, not contributing in any substantial manner to the signal of interest being developed.
Also, junctions and traps in the construction of the sensor 10, such as those generally indicated at 28, can reduce the bandspreading of the cells 12 by inducing drag and trapping effects on the test fluid 14.
Wherefore, it is the object of the present invention to overcome the aforesaid and other disadvantages of the prior art and to provide a method of improving the bandspreading and reducing the signal-to-noise ratio in such sensors by vitually eliminating the effects of junctions, traps and secondary pores in the construction.